A man has been sentenced to four years in jail in Niger in the first ever conviction for slavery in the country.

The pressure group Anti-Slavery International told the BBC the 63-year-old man was convicted of having what is known as a "fifth wife".

Men in Niger are allowed to have four wives under a local interpretation of Islamic law.

With a "fifth wife", no marriage takes place and the woman is treated solely as property.

We hope that this judgment will serve as a catalyst for more prosecutionsSarah Mathewson, Anti-Slavery International

Niger officially banned slavery in 2003 but anti-slavery organisations say thousands of people still live in subjugation.

The conviction took place in the town of Birnin Konni in south-west Niger, close to the border with Nigeria.

Anti-Slavery International says "fifth wives" are often girls of slave descent sold to wealthy men who view the purchase of young women as a sign of prestige.

The women face a lifetime of physical and psychological abuse and forced labour, the group says.

The case was taken to court by Anti-Slavery's partners in Niger, Timidria.

Sarah Mathewson, Africa Programme Co-ordinator at Anti-Slavery International, said it was "incredible" to achieve a conviction.

"It's been over 10 years since the law against slavery was passed in Niger and we've worked since then to bring perpetrators of slavery to justice," she said.

"We hope that this judgment will serve as a catalyst for more prosecutions, as we are pursuing many other cases before the courts."

In a landmark case in 2008, the West African regional body Ecowas found Niger's government guilty of failing to protect a woman from slavery. It ordered the government to pay compensation to the victim.